In processes for fabricating semiconductor devices such as integrated circuits, the surface of the semiconductor substrate or wafer is typically divided into regions where active devices and substrate embedded interconnects are to be formed, and other regions of dielectric which electrically separate these active regions. The dielectric regions are typically silicon dioxide and these regions are typically referred to as the field oxide. Many techniques for the formation of the field oxide have been proposed. One technique that is commonly used is the localized oxidation of silicon (LOCOS). In the LOCOS technique, the active regions of the silicon substrate are masked by a silicon nitride layer, while the field oxide regions are thermally oxidized to form a field dielectric region. Although the LOCOS technique is commonly used, there are deficiencies in the current technique which reduce yield or performance in the final product.
One deficiency is commonly known as the bird's beak problem, wherein the field oxide extends under the masking nitride layer to consume some of the usable active area. Additional problems routinely encountered with known field oxide formation processes include stress induced dislocations at the edges of the active regions, and the presence of a relatively nonplanar surface in or adjacent the fully formed field oxide. The nonplanar recesses or notches at the edges of the active region often degrade subsequently formed gate oxide, which can trap conductive layer residuals creating short circuit paths. Although the problems associated with thermal oxide growth are avoided by depositing the field oxide on the substrate, it is advantageous not to have the entire thickness of the field oxide above the substrate surface. Consequently, improved processes for the thermal growth of the field oxide are sought.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. Relevant technical literature includes the following articles: Oldham, et al., "Isolation Technology For Scaled MOS VLSI" IEDM 82, pages 216-219 (1982); Chiu, et al., "A Bird's Beak Free Local Oxidation Technology Feasible For VLSI Circuits Fabrication" IEEE Transactions on Electron Devices, pages 536-540 (April 1982); Chiu et al., "The Sloped-Wall SWAMI-A Defect-Free Zero Bird's Beak Local Oxidation Process For Scaled VLSI Technology" IEEE Transactions on Electron Devices, pages 1506-1511 (November 1983); Fang et al., "Defect Characteristics and Generation Mechanism In a Bird Beak Free Structure By Sidewall Masked Technique" Journal of Electrochemical Society, pages 190-196 (1983); Tsi et al., "A New Fully Recessed-Oxide (FUROX) Field Isolation Technology For Scaled VLSI Circuit Fabrication" IEEE Electron Device Letters, pages 124-126 (February 1986); Kahng et al., "A Method For Saving Planar Isolation Oxides Using Oxidation Protected Sidewalls" Journal of Electrochemical Society, pages 2468-2471 (November 1980); Chiu et al., "The SWAMI-A Defect Free And Near-Zero Bird's-Beak Local Oxidation Process And Its Application In VLSI Technology" IEDM 82, pages 224-227 (1982); Inuishi et al., "Defect Free Process Of A Bird's Beak Reduced LOCOS" Abstract No. 273 in an unknown publication, pages 40-410 (1985 or later date); Teng et al., "Optimization of Sidewall Masked Isolation Process" IEEE Journal of Solid-State Circuits, pages 44-51, (February 1985). The diversity and complexity of the various technical approaches substantiates the difficulty and importance of developing a commercially viable process for isolating active regions in a silicon substrate during the fabrication of integrated circuits.
Another solution to the bird's beak problem is proposed in U.S. Pat. Nos. 4,986,879, 4,923,563 and 5,248,350 to Lee. In these processes, a layer of silicon nitride is formed on the sidewalls of a trench formed in a substrate with masking layers thereover. The sidewalls define the field oxide regions in the substrate. Lee et al., require the silicon nitride layer to have a specified thickness, so that it will be entirely consumed as the oxide is grown in this region.
However, it is difficult to precisely control the thickness of the silicon nitride layer to ensure that it will be entirely consumed in about the same amount of time that it takes to grow the field oxide. If the silicon nitride is consumed before the desired amount of field oxide is grown, the silicon that is laterally adjacent the sidewalls is left unprotected during the remaining portions of the oxide growth. The silicon in these adjacent regions will then be oxidized, which reduces the device active area. If the silicon nitride is not totally consumed when the desired amount of field oxide is grown on the substrate, the remaining amount of silicon nitride must be removed before further processing. Accordingly, further improvement in the LOCOS process is desired.